This invention relates to a signal processing unit for use in a digital transmission system, such as a closed-circuit television system.
In a television system, a succession of picture data is transmitted to convey television pictures from a transmitting end to a receiving end. The pictures are represented by a time sequence of a large number of picture elements.
Recent development of a conference television system is remarkable. In the conference television system, a predictive coding technique has been widely used on transmitting the picture data succession. This is because it is possible to compress the picture data succession which should be transmitted from the transmitting end to the receiving end. In the conference television system using the predictive coding technique, each of the transmitting and the receiving ends comprises a signal processing unit. The signal processing unit of the transmitting end is for producing a prediction signal predictive of the picture data succession.
A conventional signal processing unit comprises two prediction circuits and a selection circuit coupled to the prediction circuits in the manner which will later be described more in detail. One of the two prediction circuits is an interframe prediction circuit for predicting each frame of the picture data succession by a preceding frame to produce an interframe prediction signal. The other prediction circuit is herein called an additional prediction circuit. At a current instant of time, the additional prediction circuit predicts a part of the picture data succession to produce an additional prediction signal by an additional part which appears in the picture data succession at another instant previous to the current instant. The selection circuit is for selecting one of the interframe and the additional prediction signals as that part of the prediction signal which should be used at the current instant.
It is to be noted here as regards the conference television system that the picture element can be classified into three groups as follows. A first group of the picture elements is representative of a stationary area, namely, a background. A second group of the picture elements is representative of a movable object, such as a person who is participating in a conference. A third group of the picture elements is representative of, for example, that part of the background which become visible as a result of movement of the movable object.
In this connection, it has been found that the conventional signal processing unit is incapable of appropriately predicting the picture data signal. This is because the additional prediction signal is only predictive of either the first group of the picture elements or the second group.